Via Ellen and LockerGnome, a law student looks at LotR.

"As a small token of your friendship Sauron asks this," he said: "that you should find this thief," such was his word, "and get from him, willing or no, a little ring, the least of rings, that once he stole. It is but a trifle that Sauron fancies, and an earnest of your good will. Find it, and three rings that the Dwarf-sires possessed of old shall be returned to you, and the realm of Moria shall be yours for ever. Find only news of the thief, whether he still lives and where, and you shall have great reward and lasting friendship from the Lord. Refuse, and things will not seem so well. Do you refuse?"
--The Fellowship of the Ring, in "The Council of Elrond"

It seems to me that's really two, maybe three separate offers. The first seems to be unambiguously an offer for a unilateral contract (to find the supposedly piddling ring for three of the Dwarf rings of power plus the estate of Moria), to be completed by performance. Dáin wouldn't want to bind himself to produce a ring; it's too risky. This seems like the straight-forward reward scenario envisioned as a prototypical offer for a unilateral contract.

A few comments on material facts: You might say that Sauron should have disclosed the Balrog living in the deeps of Moria. But the Dwarves had ancient records of Moria which probably mention this, and Sauron is old enough to imagine that the Dwarves knew. It seems silly to require disclosure of a fact which, though admittedly material, is known to both parties, even though they never actually mention it to each other. The same sort of reasoning applies to the fact that the Dwarven rings are actually tainted (although Dwarves tend to resist his power a little better than men). [Letters of Marque]